Phosphine-Coordinated Pure-Gold Clusters: Diverse ... · Phosphine-Coordinated Pure-Gold Clusters: Diverse Geometrical Structures and. . . Structurally defined molecular metal clusters

Instructions for use

Title Phosphine-Coordinated Pure-Gold Clusters: Diverse Geometrical Structures and Unique Optical Properties/Responses

Author(s) Konishi, Katsuaki

Citation Structure and Bonding, 161, 49-86https://doi.org/10.1007/430_2014_143

Issue Date 2014

Doc URL http://hdl.handle.net/2115/57678

Rights "The original publication is available at www.springerlink.com"

Type bookchapter (author version)

File Information Konishi_author version Struct. Bond .pdf

Hokkaido University Collection of Scholarly and Academic Papers : HUSCAP

https://eprints.lib.hokudai.ac.jp/dspace/about.en.jsp

Struct Bond (2014) 161: 49-86 DOl: 10.1007/430_2014_143 !"#$%&'()&*+%,'

K. Konishi (!) Faculty of Environmental Earth Science and Graduate School of Environmental Science, Hokkaido University, North 10 West 5, Sapporo 060-0810, Japan e-mail: [email protected]

Phosphine-coordinated Pure-Gold Clusters: Diverse Geometrical Structures and Unique Optical Properties / Responses

Katsuaki Konishi

Abstract Synthetic techniques, geometrical structures, and electronic absorption spectra of phosphine-coordinated pure-gold molecular clusters (PGCs) accumulated over 40 years are comprehensively collected especially for those with unambiguous X-ray crystal structures available. Inspection of the electronic absorption spectra from geometrical aspects reveals that their optical properties are highly dependent on the cluster geometries rather than the nuclearity. Recent examples of unusual clusters that show unique color / photoluminescence properties and their utilization for stimuli-responsive modules are also presented.

Keywords Abdorption spectrum Chromism Gold cluster Optical propertyPhosphine Photoluminescence Stimuli-responsive materials

Contents 1 Introduction 2 Characterization 3 Synthesis 3.1 Direct Synthesis from Gold Complex Ion or Bulk Gold 3.2 Post Synthesis 4 Geometrical Structure 4.1 Centered Polyhedral Clusters 4.2 Non-centered Polyhedral Clusters 4.3 Exo-attached Polyhedral Clusters (core+exo Type) 5 Optical Properties 5.1 Electronic Absorption Spectra 5.2 Photoluminescense 5.3 Gold Clusters as Stimuli-Responsive Chromic Modules 6 Concluding Remarks References

K. Konishi

Abbreviations Ar Aryl AZS Alizarinsulfonate Bu Buthyl DCM Dichloromethane dppb Bis(diphenylphosphino)butane dppe Bis(diphenylphosphino)propane dpph Bis(diphenylphosphino)hexane dppm Bis(diphenylphosphino)methane dppo Bis(diphenylphosphino)octane dpppe Bis(diphenylphosphino)pentane equiv Equivalent(s) Et Ethyl i-Pr Isopropyl Mes Mesityl PL Photoluminescence py Pyridyl t-Bu Tert-butyl Tol Tolyl

1 Introduction

During the last several decades, the chemistry of ligand-protected noble metal nanoparticles/clusters has progressed rapidly. Fundamental aspects have been addressed and practical uses have been found in diverse research areas including medicine, electronics, and catalysis [1–6]. The most prominent property of conventional gold particles (diameter > 2 nm) is localized surface plasmon bands, which have been widely exploited for biomedical applications [4, 7]. On the other hand, for very small clusters in the subnanometer size regime (d ~1 nm or less), such plasmon resonances disappear and discrete electronic transitions emerge which are characteristic molecular properties [8–12]. Thus, it is now well recognized that the subnano-sized clusters are virtually different from conventional colloidal particles/clusters in terms of electronic structures and properties [8]. Especially in recent years, much attention has been paid to their unique optical properties since the discovery of photoluminescence of some ultrasmall gold clusters in this size regime [13]. The reactivity and catalysis of the ultrasmall metal entities have also attracted great interest [5].

Phosphine-Coordinated Pure-Gold Clusters: Diverse Geometrical Structures and. . .

Structurally defined molecular metal clusters offer nice models to establish the structure—property relationship in the subnanometer regime. Molecular structures of numerous phosphine- and thiolate-ligated gold clusters, which form major families in cluster chemistry, have been determined. From a structural standpoint, thiolate-capped clusters, whose molecular structures were extensively elucidated just recently, are interesting but somewhat complicated because the metal cores are covered by multiple gold-thiolate staples [14–18]. On the other hand, phosphine- capped gold clusters (PGCs) have simple core-shell structures composed of the inorganic moiety (metal core) and surrounding organic ligands (shell) because of the neutral character of phosphine, thus they provide a useful platform to assess the nature of the inner gold core moieties and organic-inorganic interfaces. Further, PGCs have a long history of studies to provide a large structure library. Surveying of the Cambridge Structural Database (CSD) (2012) has identified approximately 300 crystal structures of gold-based cluster compounds.

The first example of PGC can be traced back to the pioneering work by Malatesta in 1966[19], but the structure could not be accurately defined. Later, in 1970s and early 1980s, definitive X-ray structure determinations were achieved independently mainly by a Nijmegen group [20–33] and Mingos et al. [34–43] for various PGCs with nuclearity of 5 – 13 [44–49]. Most of these clusters exploited monodentate phosphines as the ligands, revealing the general trend of the geometrical preference in the formation of ultrasmall clusters under unconstrained conditions. Through these studies, fundamental structure and bonding of this class of compounds were comprehensively established and were also interpreted theoretically [50–52]. Thereafter, major interests in ligand-coordinated gold clusters moved to heterometallic, higher-nuclearity (e.g., Au39, Au55), and thiolate-capped families. However, the interests in ultrasmall molecular PGCs have recently revisited [8, 53–55] in relation to the advances of structural chemistry of thiolate-capped clusters. Further, the use of state-of-the-art analytical instruments together with the advanced computation technology has opened the door to unusual cluster species.

Asides from conventional clusters, there is another line of studies on "Au(I)" clusters which do not contain formally zerovalent gold atoms [56, 57]. Further, there are extensive studies on clusters whose skeletons are constituted primarily by gold but contain other metal elements [58, 59] or encapsulated main group element (e.g., C, N) [60]. This chapter does not cover them and focuses on homoleptic and heteroleptic phosphine-coordinated pure-gold cluster compounds with formally mixed-valence character (formal averaged oxidation number lies between 0 and 1). Thus the general formula can be described as [AuN(PR3)mXs]z+ (N > z), in which tertiary phosphines (PR3) serve as primary protecting ligands and additional ligands (X), such as halide and thiolate anions, sometimes coordinate to the surface metals as sub-ligands. This chapter first discusses the characterization and synthetic techniques of PGCs and then surveys the structural features and electronic absorptions / photoluminescence properties. Since the studies before 1990 established by the first generation have been already comprehensively summarized in literatures [25, 34, 35] and also in this book, emphases are placed on the recent advances in the clusters with unusual structures. In the last part of this chapter, the application of PGCs for stimuli-responsive chromic modules is also presented.

K. Konishi

2 Characterization

Identification and characterization of PGCs have been achieved in terms of nuclearity (size), molecular formulae and geometrical structures by various analytical techniques. Among them, single-crystal X-ray crystallography is still the most definitive technique to elucidate the geometrical structures, but it is limited to the clusters that give high-quality crystals suitable for the structure determination. Recently mass spectrometry coupled with soft-ionization techniques (e.g., electro-spray ionization (ESI), matrix-assisted laser desorption ionization (MALDI)) are found to be powerful tools to obtain direct information about the molecular mass and formulae of cluster species. ESI-mass spectroscopy is especially useful since it can analyze solution samples, allowing the direct analyses of the reaction mixtures. However, one must note that the observed peak, especially before the purification procedure, may be a result of fragmentation/recombination in the gas phase, and the results sometimes strongly depend on the measurement conditions (e.g., voltage). Further, mass spectra give only molecular formula information so the geometrical structures must be deduced with the aid of other characterization methods. 1H and 31P NMR spectra are useful to investigate the dynamics of the coordinated ligands on the cluster surface, but are not suitable to obtain definitive molecular structures. TEM has been used to estimate the size and nuclearity of large clusters but the application for small clusters is limited because of the resolution limitation. Electronic absorption (UV–visible) spectroscopy is classical but has been most conveniently utilized as a characterization tool. The spectral features of cluster species in the subnanometer-regime are unique to individual clusters and are sensitive to the structures/nuclearity of the cluster compounds, so it is possible to identify unknown species based on the spectral pattern of structurally identified authentic samples. Further, since the spectral profiles are closely correlated with the electronic structures, they are useful to understand the geometry- or nuclearity (size)-dependent profiles of PGCs. This aspect will be discussed in Sect. 5.

3 Synthesis

Generally, PGCs are synthesized in homogeneous solution either by the reduction of ionic gold sources (e.g., gold(I) phosphine complex, tetrachloroaurate) or by the post-synthetic methods involving the growth and/or etching of preformed PGCs. Other methods have also been known, but most of the synthetic procedures reported to date fall into these two categories. The representative examples were summarized in Schemes 1, 2, 3, 4, 5, 6, 7, 8, and 9 in terms of the reaction types.


Scheme 1 Selected examples of PGC syntheses via hydride reduction of gold(I) phosphine complexes

Au(PAr3)X Au11(PAr3)7X3 (23 - 27)

(P^P = MeO-BIPHEP, BINAP)

(X = Cl, I, SCN, SR; Ar = Ph, 4-OMe-C6H4, 4-MeC6H4)

(b)

Au2(P^P)Cl2

Au(PAr3)NO3NaBH4

EtOH[Au9(PAr3)8]3+ (14 - 16)(a)

[Au11(P^P)4Cl2]Cl

Au2(dppe)Cl2 [Au11(dppe)6]3+ (32)

Au(PCy2Ph)(NO3)

Au2(dppo)Cl2 Au22(dppo)6 (37)

(c)

Au2(dppp)Cl2 [Au11(dppp)5]3+ (30)

Au(PPh3)Cl + depp [Au11(depp)4Cl2]+

Au(PPh(4-Py)2)Cl [Au20(PPh(4-Py)2)10Cl4]2+ (36)

[Au10(PCy2Ph)6Cl3]+ (18)

(d)

Au(PPh3)Cl + dpppt-BuNH2·BH3

CHCl3

(e)

(f)

(g)

(h)

(i)

(j)

NaBH4

EtOH

NaBH4

EtOH

NaBH4

EtOH

NaBH4

EtOH

t-BuNH2·BH3

CHCl3

NaBH4

EtOH

NaBH4

EtOH

NaBH4

EtOH

[Au11(dppp)5]3+ (30)

(Ar = Ph, 4-OMe-C6H4, 4-MeC6H4)

K. Konishi

Scheme 2 Syntheses of Au9 and Au11 clusters by the reduction with zero-valent titanium reagent

Scheme 3 Examples of PGC syntheses via etching reactions

3.1 Direct Syntheses from Gold Complex Ion or Bulk Gold

3.1.1 Borohydride or Borane Reduction of Gold(I) Phoshine Complex

The borohydride reduction of monophosphine gold(I) complex (Au(PR3)X) in ethanol is the original recipe and has been most widely used as a conventional method, which generally yields Au9 [27, 37, 38, 61, 62] or Au11 species[19, 44–48, 63–65]. NaBH4 is commonly exploited as the reducing agent, because of the ease in handling. From gold(I) triarylphosphine complex (Au(PAr3)3X) with a strongly coordinating anion (X = Cl, I, SCN, S-4-py), neutral deca-coordinated undecagold clusters Au11(PAr3)7X3 (e.g., 23–27, Table 1) are preferentially formed (Scheme 1a), whereas the formation of cationic octa-coordinated nonagold cluster [Au9(PAr3)8]3+ (e.g., 14–16) is preferred when X is NO3 with a weak coordinating character (Scheme 1a). The steric hindrance and the presence of additional coordination sites in the phosphine ligand affect the extent of the cluster growth processes.

Au(PPh3)ClTi(!-C6H5CH3)2 [Au9(PPh3)8]3+ (14)

Au(PMePh2)Cl [Au11(PMePh2)10]3+ (31)

Au(PMe2Ph)Cl [Au11(PMe2Ph)10]3+ (31’)

Ti(!-C6H5CH3)2

Ti(!-C6H5CH3)2

PPh3[Au9(PPh3)8]3+ (14) [Au8(PPh3)8]2+ (8) + [Au(PPh3)2]+

[Au13(PMe2Ph)10Cl2]3+ PMe2Ph [Au11(PMe2Ph)10]3+ (31’)

[Au9(PPh3)8]3+ (14) [Au8(PPh3)7]2+ (7)

[Au8(PPh3)8]2+ (8)

S

CN

S

NC

[Au6(PPh3)6]2+ (3)K[Ag(CN)2]

[Au9(PPh3)8]3+ (14) [Au8(dppp)4]2+ (9)dppp

[Au6(dppp)4]2+ (4)dppp

(b)

(a)

(c)

(d)

(e)

+ 2 AuCl(PMe2Ph)2


.

Scheme 4 Examples of PGC syntheses via growth (aggregation) reactions

Scheme 5 Examples of simple ligand-exchange reaction

Scheme 6 Two-step synthesis of icosahedral Au13 cluster via HCl-promoted growth/etching

[Au8(PPh3)8]2+ (8) + Au(NO3)(PPh3) [Au9(PPh3)8]3+ (14)

[Au11(PMe2Ph)10]3+ (31’) + 2 AuCl(PMe2Ph)2

[Au8(dppp)4Cl2]2+ (11)

[Au13(PMe2Ph)10Cl2]3+ (33)

[Au6(dppp)4]2+ (4) + 2 AuCl(PPh3)

[Au9(PPh3)8]3+ (14) Au10(PPh3)3(C6F5)4 (20)[Au(C6F5)]–

[Au9(PPh3)8]3+ (14)Cl–

[Au11(PPh3)8Cl2]+ (29)

[Au11(PMe2Ph)10]3+ (31’)

[Au9(PPh3)8]3+ (14)4-pyS–

Au11(PPh3)7(4-pyS)3 (27)

[Au13(PMe2Ph)10Cl2]3+ (33)Cl–

[(Au(PMes)3)3O]+CO

[Au8(PMes3)6]2+ (10)

(b)

(a)

(c)

(d)

(e)

(f)

(g)

(h)

(i) [Au11(PPh3)8Cl2]+ (29) EtSH [Au25(PPh3)10(SEt)5]2+ (39)

dpph[Au9(PPh3)8]3+ (14) [Au9(dpph)4]3+ (17)

Au11(PAr3)7(SCN)3 dppp

[Au11(dppp)5]3+ (30)

cluster formation

Au2(dppe)Cl2polydisperse mixture(Au9, Au11, Au12, Au13, Au15)

NaBH4 conc.HCl(step 1) (step 2)

nuclearity convergence

[Au13(dppe)5Cl2]3+

(34)

(DCM / EtOH) (EtOH)

K. Konishi

Scheme 7 Formation of two-types of Au13 clusters

Scheme 8 Nucleophilic addition to [Au8(dppp)4]2+ (9) accompanying the oxidative skeletal isomerization

Scheme 9 Schematic illustration of the two-electron redox processes between [Au9(PPh3)8]3+ (14) and [Au9(PPh3)8]+

Reaction of Au(PCy2Ph)(NO3) with NaBH4 affords [Au10(PCy2Ph)6Cl3]+ (18) (c) [39]. Recently, Wang et al. reported that the reduction of AuCl(PPh(4-py)2) with NaBH4 yields a larger cluster with an ionic formula [Au20(PPh(4-py)2)10Cl4]2+ (36), in which the pyridyl groups (py) additionally coordinate with the surface gold atoms (d) [66]. Diborane can be also utilized as the reducing agent. Reduction of Au(PPh3)Cl in benzene gives Au55(PPh3)12Cl6 [67], but its single-crystal structure is not available yet. Diphosphines (P^P) have also been used as the ligands in the borohydride reduction of chlorogold(I) complex ions (Scheme 1e–j). Similarly to the reduction of monophosphine complex, this reaction system generally affords decacoordinated

[Au13P8Cl4]+

m = 2(dppe)

m = 3, 4, 5(dppp, dppb, dpppe)

[Au13P10Cl2]3+

Ph2P Au

AuPh2P

(CH2)m

Cl

Cl

1) NaBH42) HCl

Au2P2Cl2

[Au13(dppe)5Cl2]3+ (34) [Au13(dppp)4Cl4]+ (41) [Au13(dppb)4Cl4]+ (42) [Au13(dpppe)4Cl4]+ (43)

(a)

(b)

[Au8(dppp)4]2+ (9)2 Cl–

[Au8(dppp)4Cl2]2+ (11)

[Au8(dppp)4]2+ (9)2 RC!C–

[Au8(dppp)4(C!CR)2]2+ (12)

– 2e–

– 2e–

(Au8)2+ (Au8)4+

(Au9)3+ (Au9)+

–2e–

+2e–


Au11L10-type (L1/4phosphine or Cl) clusters predominantly, but the ligand ratio (P:Cl) is different. For example, in the reduction of Au(PPh3)Cl with t-BuNH2·BH3 in chloroform in the presence of C3-bridged diphosphine (dppp), the predominant formation of Au11P10-type cluster ([Au11(dppp)5]3+, 30) is detected in the mass spectra (j) [68]. We have also found that the simple NaBH4 reduction of Au2(dppp)Cl2 in ethanol gives 30 as the main cluster product (f) [69]. On the other hand, the mass spectrometric analyses of the reduction system Au(PPh3)Cl / t-BuNH2·BH3 coupled with non-phenyl type C3-bridged diphosphine ligand (Et2P(CH2)3PEt2, depp) showed the preferential formation Au11P8Cl2-type cluster [Au11(depp)4Cl2]+ (i) together with [Au12(depp)4Cl3]+ and [Au13(depp)4Cl3]+ [70]. Such Au11P8Cl2-type clusters are also obtained as the main cluster products in the NaBH4 reduction of dinuclear chlorogold complexes of chiral diphosphines BINAP [71] and MeO-BIPHEP [72] in ethanol (h), which are identified by mass and absorption spectra but their single-crystal X-ray structures are not available.

On the other hand, the use of particular diphosphines results in the formation of different cluster species due to the steric and chelating effects of diphosphines. For example, he above Au(PPh3)Cl / tBuNH2·BH3 system coupled with C5- (dpppe, Ph2P(CH2)5PPh2) or C6-bridged (dpph, Ph2P(CH2)6PPh2) diphosphine gives [Au8(P^P)4]2+ and [Au10(P^P)4]2+ as the main cluster products. Wang et al. have recently reported that the NaBH4 reduction of chlorogold(I) complex of C8-bridged diphosphine (dppo) affords a larger cluster Au22(dppo)6 (37, Scheme 1g) [73]. We have also shown that the use of C2-bridged diphosphine (dppp) for the NaBH4 reduction of chlorogold(I) complex in ethanol results in the formation of a Au11 species, which however accommodate a different number of coordinating phosphine ligands (Au11L12 type) (e) [74]. The ionic formula is [Au11(dppe)6]3+ (32), which exhibits an unusual green color. In this reaction, the cluster products are strictly dependent on the reaction medium. When the same reaction is conducted in CH2Cl2 / E tO H (50 /50 v /v ) , t he ESI -m ass spec t rum g ives m u l t ip l e s igna l s

n = 1: dppmn = 2: dppen = 3: dpppn = 4: dppbn = 5: dpppen = 6: dpphn = 8: dppo

xy-xantphos(Ar = 3,5-xylyl)

dppa (S)-MeO-BIPHEP

depp

Ph2P PPh2

OP P

Ar

Ar

Ar

Ar

PPh2MeOMeO

PPh2

Ph2P CH2 PPh2n

Et2P CH2 PEt23

K. Konishi

assignable to [Au9(dppe)5]3+, [Au11(dppe)5]3+, [Au12(dppe)5Cl]3+, [Au13(dppe)5Cl2]3+ and [Au15(dppe)6ClH]3+ (Scheme 6, vide infra) [75]. On the other hand, the same reduction system from the C3-brdiged complex (Au2(dppp)2Cl2) in CH2Cl2 / EtOH (50/50 v/v) affords ESI-mass signals assignable to higher nuclearity clusters [Au38(dppp)9Cl2]4+ and [Au39(dppp)Cl3]4+ [76]. These results imply that the cluster growth processes are not only governed by the intrinsic stability of the gold core but also notably affected by the steric and cheating effects of multidentate ligands and reaction conditions.

3.1.2 Other Methods

Other reducing agents have been also utilized. The utilization of Ti(!-C6H5CH3)2 developed by Mingos et al. affords [Au9(PAr3)8]3+ (14, 16) from AuCl(PPh3) (Scheme 2) [36]. The same reduction system from Au(PMe2Ph)Cl and Au(PMePh2)Cl) results in the formation of deca-phosphine-coordinated Au11 cluster compounds having no Cl ligands on the cluster surface ([Au11(PMePh2)10]3+ (31) and [Au11(PMe2Ph)10]3+ (31’)[34, 42]. Recently, [Au6(xy-xantphos)3]Cl (5·Cl) was isolated by recrystallization from the reaction mixture of Au(xy-xantphos)Cl and PhMe2SiH [77]. HAuCl4 instead of gold phosphine complex is usable as the starting gold source. Upon reduction of HAuCl4 by NaBH4 in the presence of trioctylphosphine in aqueous THF, [Au13(POct3)8Cl4]+ is obtained as a minor product [76]. [Au39(PPh3)14Cl6]Cl2 (40·Cl), which is, to date, the largest PGC characterized by single-crystal X-ray diffraction, was synthesized by the reduction of HAuCl4 by NaBH4 [78]. Recently, [Au24(PPh3)10(SC2H4Ph)5X2]+ (38) was synthesized by Jin et al. through the NaBH4 reduction of HAuCl4 based on the Brust’s two-phase method [79]. Several other gold sources have been also utilized. [Au7(PPh3)7]+ (6) can be prepared by the reaction of gold vapor with PPh3 in toluene [31]. Azido complex AuN3(PPh3) is also usable as a precursor, which affords [Au8(PPh3)8]2+ (8) upon photolysis in THF [80].

3.2 Post Synthesis

As noted in 3.1, Au9 and Au11 clusters are frequently found as the primary products in the direct PGC synthesis from gold(I) precursors and thus appear to be relatively stable compounds. However, due to the flexible nature of the cluster skeletons and lability of Au-Au interactions, they can be easily converted into different cluster species via post-synthetic aggregation-mediated growth and degradation-mediated etching processes, thus acting as useful intermediates for a variety of PGC compounds.


3.2.1 Etching Reactions

Several etching (degradation) reactions of monophosphine-protected Au9 and Au11 clusters upon treatment with free phosphines have been reported. [Au8(PPh3)8]2+ (8) can be obtained by the reaction of [Au9(PPh3)8]3+ (14) with free PPh3, which accompanies the liberation of Au(PPh3)+ (Scheme 3a) [20, 49]. In a similar fashion, [Au11(PMe2Ph)]3+ (31’) can be synthesized from [Au13(PMe2Ph)10Cl2]3+ (33) in the presence of free PMe2Ph (b) [34]. Reaction of [Au9(PPh3)8] 3+ (14) with sodium maleonitriledithiolate (MNTNa2:Na2S2C2(CN)2) affords hepta-cooridnated Au8 cluster ([Au8(PPh3)7]2+, 7) (c) [41], which can be also obtained by the phosphine abstract reaction of 8 by [RhCl(cyclooctene)2]2 [30]. The reaction of [Au8(PPhR2)8](NO3)2 (R = Ph, 8·NO3) in the presence of K[Ag(CN)2] affords edge-sharing bitetrahedral [Au6(PPhR2)8]2+ (R = Ph (3), 2-MeC6H4, Cy) (d) [40].

The etching reactions during the ligand exchange reaction with diphosphine have been also reported. The treatment of [Au9(PPh3)8](NO3)3 (14·NO3) with excess free dppp results in the formation of core+exo type [Au6(dppp)4]2+ (4) (e) [26]. As an intermediate cluster species in this reaction, [Au8(dppp)4]2+ (9) have been isolated and identified by mass spectrometry and single-crystal X-ray diffraction studies [81].

3.2.2 Growth Reactions

The addition reaction of mononuclear gold species to preformed cluster species results in the growth of the cluster nuclearity. The reaction of [Au8(PPh3)8]2+ (8) with Au(NO3)(PPh3) leads to the formation of [Au9(PPh3)8](NO3)3 (14·NO3) (Scheme 4a) [21, 31]. Likewise, [Au11(PMe2Ph)10]3+ (31’) grows to [Au13Cl2(PMe2Ph)10]3+ (33) by the addition of AuCl(PMe2Ph) (b) [34]. The treatment of [Au6(dppp)4](NO3)2 (4·NO3) with Au(PPh3)Cl results in the formation of [Au8(dppp)4Cl2]2+ (11) (c) [81]. An organometallic cluster Au10(PPh3)3(C6F5)4 (20) can be obtained by the reaction of [Au9(PPh3)8]3+ (14) with (NBu4)[Au(C6F5)] (d) [82].

The reactions with nucleophilic agents are known to induce aggregation reactions between several cluster molecules. This reaction is unpredictable, but sometimes useful for the syntheses of higher nuclearity clusters. In the presence of Cl– with coordinating capability, [Au11(PMe2Ph)10]3+ and [Au9(PPh3)8]3+ (14) are converted into [Au13Cl2(PMe2Ph)10]3+ (33) [36] and [Au11(PPh3)8Cl2]+ (29) [21, 83], respectively (Scheme 4e and g). The reaction of [Au9(PPh3)8] 3+ (14) with 4-pyridinethiol in alkaline methanol results in the formation of Au11(PPh3)7(4-pyS)3 (f) [84]. Shichibu et al. reported the synthesis of [Au25(PPh3)10(SEt)5Cl2]2+ (39) by the reaction of [Au11(PPh3)8Cl2]Cl (29·Cl) and ethanethiol [85]. For other examples, the reduction of trinuclear oxonium salt [Au(PMes)3]3O]+ by CO affords hexa-coordinated Au8 cluster [Au8(PMes3)6]2+ (10) (Scheme 4h) [86]. The decomposition of trinuclear hydradizo complex [(AuPPh3)3NNR2]BF4 leads to the

K. Konishi

formation of [Au6(PPh3)6]2+ (3) [87], which is isostructural to that obtained by the silver-mediated etching reaction of [Au8(PPh3)8]2+ (8).

3.2.3 Ligand Exchange Reactions

As noted above, the reaction of once generated clusters with free ligands often results in the nuclearity alteration through the etching and growth of the cluster cores, but simple exchange reactions with the retention of the cluster nuclearity and structure take place when appropriate free ligands are utilized. For example, [Au11(dppp)5]3+ (30) is obtained by the reaction of Au11(P(4-ClC6H4)3)7(SCN)3 with dppp (Scheme 5) [24]. Likewise, the reaction of dpph with [Au9(PPh3)8](NO3)3 (14·NO3) with four equiv of dpph affords [Au9(dpph)4]3+ (17) quantitatively [88].

3.2.4 Growth/Etching During Crystallization Process

Due to the soft potential surface of gold clusters, crystallization process sometimes allows the formation of unexpected cluster species that is different from the original cluster in the mother liquors. For example, crystallization of [Au8(PPh3)7] [S2C2(CN2)2] from CH2Cl2/MeCN results in the formation of the crystals of Au10(PCy3)7(MNT) (19) [41]. The formation of crystals containing Au7 and Au9 clusters have been reported when the polyoxometalate and fullerene salts of [Au8(PPh3)8]2+ are crystallized [61, 89]. Very recently, crystals of Au14(PPh3)8(NO3)4 (35) have been isolated during the crystallization process in the Au9(PPh3)8(NO3)3 (14·NO3) synthesis by the Au(PPh3)NO3/NaBH4 system [90].

3.2.5 Nuclearity Convergence Through Simultaneous Etching/Growth

We have recently found that HCl has a unique capability to promote the etching and growth of PGCs [75, 76]. Through the simultaneous etching/growth processes, a polydisperse mixture of several dppe-coordinated clusters, which was formed by the NaBH4 reduction of Au2(dppe)Cl2 in CH2Cl2/EtOH, is exclusively converted into [Au13Cl2(dppe)5]3+ (34) upon treatment with aqueous HCl (12 M) in EtOH (Scheme 6). This method is practically useful, allowing gram-scale preparation of icosahedral Au13 cluster (total yield exceeds 70%). Other protonic acids such as acetic acid and sulfuric acid, and tetraethyl ammonium chloride also served as promoters but are much inferior to hydrochloric acid, indicating that the effective nuclearity convergence is a result of growth/etching under thermodynamic conditions by the cooperation of acidic proton and chloride anion. The chelation effect of the diphosphine ligands plays a critical role in keeping the cluster structure and also affects the composition of the coordinating ligands of the final cluster products. With monophosphine ligand, the intermediate cluster species does not survive


the strongly acidic condition, while the use of diphosphine ligands with a longer alkyl bridge (dppp, dppb, dpppe) results in the preferential formation of [Au13Cl4(P^P)4]3+ (41–43) (Scheme 7).

3.2.6 Redox Reations

As mentioned in the above paragraphs, the addition of nucleophiles such as Cl- to monophosphine-coordinated clusters promotes the aggregation-induced growth of the cluster core. However, we recently found that the nucleophilic reaction towards [Au8(dppp)4]2+ (9) proceeds with the retention of the nuclearity but accompanies two-electron autoxidation and rearrangement of the cluster skeleton[91]. For instance, the reactions with nucleophiles such as halide and acetylide anions afford [Au8(dppp)4X2]2+ (X = Cl (11), X = C!CPh (12)), where the charge of the Au8 skeleton is altered from 2+ to 4+ (Scheme 8). In this relation, electrochemical two-electron redox processes between [Au9(PPh3)8]3+ (14) and [Au9(PPh3)8]+ which accompany the skeletal rearrangement from toroidal to spherical, have also been reported (Scheme 9) [32, 92]. The large structural change may reflect the soft potential energy surface of this class of compounds.

3.2.7 Summary

A variety of PGCs can be synthesized in good yields when the appropriate reagents and pathways are used. However the cluster products are generally unpredictable and vary significantly with the subtle difference of synthetic parameters (methods, conditions, reagents, and so on).

4 Geometrical Structure

As noted in Sect. 2, single-crystal X-ray crystallography is the sole definitive method to elucidate the 3-dimensional structural features. PGCs with a nuclearity of 5, 6, 7, 8, 9, 10, 11, 13, 14, 20, 22, 24, 25 and 39 have been structurally identified. For selected examples, geometrical descriptions and Au-Au bond distances are summarized in the nuclearity order (Tables 1 and 2). Centered polyhedral geometries are generally favored because the center-to-peripheral interactions effectively enhance the stability of cluster skeleton. The majority of the clusters with N " 8 falls into this category. However, the use of sterically hindered ligands or chelating diphosphines sometimes allows the generation of exceptional structures that cannot be described only with polyhedra.

K. Konishi

Tabl

e 1

Geo

met

rical

stru

ctur

es o

f cry

stallo

grap

hica

lly d

efin

ed P

GCs

with

nuc

lear

ity (N

) of 5

– 1

3

Coun

ter a

nion

G

eom

etry

A

u-A

u di

st. (Å

) Re

fere

nces

(N

= 5

)

[A

u 5(d

ppm

) 3(Ph

2PCH

PPh 2

)]2+ (1

) N

O3

Tetra

hedr

on +

1 e

xo

2.70

2-3.

013

[48]

(N

= 6

)

[A

u 6(P

Ph2C

6H4) 4

]2+ (2

) Sb

F 6

Oct

ahed

ron

2.72

1-3.

155

[93]

[A

u 6(P

Ph3) 6

]2+ (3

) N

O3

Edge

-sha

ring

bite

trahe

dron

2.

652-

2.83

9 [4

0]

[Au 6

(dpp

p)4]2+

(4)

NO

3 Te

trahe

dron

+ 2

exo

2.

630-

2.92

3 [2

6]

[Au 6

(xy-

xant

phos

) 3]+ (5

) Cl

Fa

ce-s

harin

g tri

tetra

hedr

on

2.65

3-3.

104

[77]

(N

= 7

)

[A

u 7(P

Ph3) 7

]+ (6)

OH

Pe

ntag

onal

bip

yram

id

2.58

2-3.

007

[31,

94]

C 6

0 Pe

ntag

onal

bip

yram

id

2.56

1-3.

092

[89]

(N

= 8

)

[A

u 8(P

Ph3) 7

]2+ (7

) N

O3

Capp

ed c

ente

red

chai

r a 2.

629-

2.94

2 [2

2]

[Au 8

(PPh

3) 8]2+

(8)

AZS

Ca

pped

cen

tere

d ch

air a

2.68

9-2.

938

[49]

PF6

Capp

ed c

ente

red

chai

r a 2.

634-

2.96

0 [2

0]

Si

Mo 1

2O40

Ca

pped

cen

tere

d ch

air a

2.

650-

2.90

9 [9

5]

[Au 8

(dpp

p)4]2+

(9)

NO

3 Ed

ge-s

harin

g tri

tetra

hedr

on

2.61

6-2.

752

[81]

[A

u 8(P

Mes

3) 6]2+

(10)

BF

4 Te

trahe

dron

+ 4

exo

2.

616-

2.75

2 [8

6]

[Au 8

(dpp

p)4C

l 2]2+

(11)

PF

6 Ed

ge-s

harin

g bi

tetra

hedr

on +

2 e

xo

2.60

7-2.

896

[81]

[A

u 8(d

ppp)

4(C!C

Ph ) 2

]2+ (1

2)

NO

3 Ed

ge-s

harin

g bi

tetra

hedr

on +

2 e

xo

2.64

8-3.

072

[91]

[A

u 8(d

ppa)

4Cl 2]

2+ (1

3)

ClO

4 Ed

ge-s

harin

g bi

tetra

hedr

on +

2 e

xo

2.61

9-3.

052

[96]

(N

= 9

)

[A

u 9(P

Ph3) 8

]3+ (1

4a)

NO

3 Bi

capp

ed c

ente

red

chai

r (bu

tterfl

y) a

2.67

5-2.

880

[62]

PW12

O40

Bi

capp

ed c

ente

red

chai

r (bu

tterfl

y) a

2.68

8- 2

.926

[6

1]

[Au 9

(PPh

3) 8]3+

(14b

) PW

12O

40

Cent

ered

cro

wn

2.66

2-2.

806

[61]

PMo 1

2O40

Ce

nter

ed c

row

n 2.

661-

2.81

0 [9

5]

[Au 9

(P{p

-Tol

} 3) 8]

3+ (1

5)

PF6

Bica

pped

cen

tere

d ch

air (

butte

rfly)

a

2.68

6-2.

891

[27]

[A

u 9(P

{p-M

eOC 6

H4}

3) 8]3+

(16a

)

BF4

Ce

nter

ed c

row

n

2.

651-

2.83

8 [3

7]

N

O3

Cent

ered

cro

wn

2.65

9-2.

843

[38]


a Can

be a

lso d

escr

ibed

as a

der

ivat

ive

or su

bstru

ctur

e of

icos

ahed

ral A

u 13

[Au 9

(P{p

-MeO

C 6H

4}3) 8

]3+ (16b

) N

O3

Bica

pped

cen

tere

d ch

air (

butte

rfly)

a 2.

689-

2.89

9 [3

8]

[Au 9

(dpp

h)4]3+

(17)

PW

12O

40

Cent

ered

cro

wn

2.64

3-3.

249

[88]

(N

= 1

0)

[Au 1

0(PPh

Cy2) 6

Cl3]+ (1

8)

NO

3 Tr

icap

ped

cent

ered

cha

ir (toro

idal

)a 2.

666-

2.94

1 [3

9]

Au 1

0(PPh

3) 7(M

NT)

2 (19

) -

Tric

appe

d ce

nter

ed c

hair (to

roid

al)a

2.56

7-3.

055

[41]

A

u 10(P

Ph3) 5

(C6F

5) 4 (20)

-

Tric

appe

d ce

nter

ed c

hair (to

roid

al) a

2.

639-

2.96

4 [8

2]

Au 1

0(PPh

3) 8Cl

(NCO

) (21

) -

Capp

ed c

ente

red

squa

re a

ntip

rism

(sph

eric

al)

2.63

5-3.

174

[97]

[A

u 10(P

Ph3) 8

Cl]+ (2

2)

PF6

Capp

ed c

ente

red

squa

re a

ntip

rism

(sph

eric

al)

2.63

7-3.

151

[98]

(N

= 1

1)

Au 1

1(PPh

3) 7Cl

3 (23

) -

Tetra

capp

ed c

ente

red

chai

ra 2.

608-

2.96

8 [6

5]

Au 1

1(PPh

3) 7I 3

(24)

-

Tetra

capp

ed c

ente

red

chai

ra 2.

634-

3.19

3 [4

5]

Au 1

1(P{p

-FC 6

H4}

3) 7I 3

(25)

-

Tetra

capp

ed c

ente

red

chai

ra 2.

670-

3.18

4 [4

7]

Au 1

1(P{m

-CF 3

C 6H

4}3) 7

Cl3 (26)

-

Tetra

capp

ed c

ente

red

chai

ra 2.

663-

3.12

6 [9

9]

Au 1

1(PPh

3) 7(4

-pyS

) 3 (27)

-

Tetra

capp

ed c

ente

red

chai

ra 2.

629-

3.12

2 [8

4]

[Au 1

1(PPh

3) 7(C

NiP

r) 2I]2+

(28)

PF

6 Te

traca

pped

cen

tere

d ch

aira

2.67

0-3.

127

[33]

[A

u 11(P

Ph3) 8

Cl2]+ (2

9)

W6O

19

Tetra

capp

ed c

ente

red

chai

ra 2.

625-

3.18

5 [8

8]

[Au 1

1(dpp

p)5]3+

(30)

SC

N

Tetra

capp

ed c

ente

red

chai

ra 2.

669-

3.08

8 [2

9]

[Au 1

1(PM

ePh 2

) 10]3+

(31)

C 2

B 9H

12

Capp

ed c

ente

red

squa

re a

ntip

rism

2.

646-

3.04

8 [4

2]

[Au 1

1(dpp

e)6]3+

(32)

Sb

F 6

Butte

rfly

Au 9

+ 2

exo

2.

608-

3.21

6 [7

4]

(N =

13)

[A

u 13(P

Me 2

Ph) 10

Cl2]3+

(33)

PF

6 Ic

osah

edro

n 2.

715-

2.95

5 [3

6]

[Au 1

3(dpp

e)5C

l 2]3+

(34)

Sb

F 6

Icos

ahed

ron

2.69

6-2.

974

[75]

K. Konishi

4.1 Centered Polyhedral Clusters

4.1.1 N # 13

Crystal structures of PGCs with 8 # N # 13 were extensively studied before 1990, and the general preference centered polyhedral geometries has been almost established. The majority of these centered clusters have skeletal structures based on centered icosahedron, which can also be described as capped centered hexagonal chairs. Such icosahedron-based structures are found for a series Au8L7, Au8L7, Au9L8, Au10L9, and Au11L10 clusters, which finally complete with the icosahedral Au13L12-type clusters (Table 1). Representative examples are shown in the structures of 7 (Au8L7), 8 (Au8L7), 18 (Au10L9), 23 (Au11L10), 30 (Au11L10), and 34 (Au13L12) (Fig. 1). The exceptional structures that cannot be described based on icosahedron are also found in several cases. For example, Mingos et al. reported green and golden-brown crystals of [Au9(P(4-MeOC6H4)3)8](NO3)3 (16·NO3), and showed that the former has a bicapped chair D2h geometry derived from icosahedron whereas the latter has a centered crown geometry [38]. The interconversion between the two forms seems easy since they share a common structure in solution. The crown geometries have been also found in the several polyoxometalate salts of Au9P8 clusters. Jansen et al. recently reported that the appropriate choice of recrystallization solvents allows the selective formation of either of the two skeletal geometries of 14·PW12O40 (Fig. 2) [61]. As other examples of deviation from icosahedron-derived structures, capped centered square antiprism (approximately D4d symmetry) geometries have been reported for [Au11(PMePh2)10]3+ (31) (Fig. 1) [42], which is obviously different from the icosahedron-based structures with approximately C3v symmetry found in the conventional Au11(PAr3)10 clusters for various combinations of phosphine, sub-ligands, and counter anion [28, 29, 33, 47, 65, 84, 88, 99] (Table 1 and 23 and 30 in Fig. 1). This geometry can also be viewed as a capped crown, which is also found for Au10L9 clusters (22 in Fig. 1) [98]. As mentioned in previous papers and reviews [35, 42], the center-to-peripheral radial bond distances are shorter than the peripheral bond distances, indicating the crucial contribution of radial bonding to the stability of cluster skeleton. The overall shape of these clusters can be categorized into toroidal (ellipsoidal, 2D) and hemispherical/spherical (3D), which have been accounted for in terms of polyhedral electron counting and molecular orbital calculations [34, 51].

4.1.2 Higher-Nuclearity Clusters

Several examples of higher-nuclearity centered clusters have appeared recently and their structures are shown in Fig. 3. Simon et al. showed that the crystal structure of [Au14(PPh3)8(NO3)4] (35) exhibits a unique geometry, in which two face-sharing tetrahedron dimers are connected by four AuNO3 units [90]. The distance between


Table 2 Geometrical structures of higher nuclearity PGCs with N > 13

Fig. 1 Single-crystal X-ray structures of [Au8(PPh3)7]2+ (7), [Au8(PPh3)8]2+ (8), [Au10(PPhCy2)6Cl3]+ (18), [Au10(PPh3)8Cl]+ (22), Au11(PPh3)7Cl3 (23), [Au11(dppp)5]3+ (30), [Au11(PMePh2)10]3+ (31) and [Au13(dppe)5Cl2]3+ (34). For counter ions, (see Table 1). For 7, 8, 18, 23, 30, and 34, the gold core units can be described as capped chairs (icosahedron derivatives), whereas those of 22 and 31 can be described as D4d bicapped crowns

[Au11(dppp)5]3+ (30)Au11(PPh3)7Cl3 (23)

[Au10(PCy2Ph)6Cl3]+ (18)

[Au10(PPh3)8Cl]+ (22)

[Au8(PPh3)8]2+ (8)[Au8(PPh3)7]2+ (7)

[Au11(PMePh2)10]3+ (31) [Au13(dppe)5Cl2]3+ (34)

Cluster formula

Counter anion

Description

Au-Au dist. (Å)

References

Au14(PPh3)8(NO3)4 (35) - Trigonal bipyramid dimmer linked by 4 AuNO3

2.582-2.984 [90]

[Au20(PPh{4-Py}2)10Cl4]2+ (36)

Cl Edge-sharing icosahedral Au11 dimer

2.644-3.189 [66]

Au22(dppo)6 (37) - Icosahedral Au11 dimer 2.631-3.227 [73] [Au24(PPh3)10(SC2H4Ph)5X2]+ (38)

Unknown Incomplete icosahedral Au12 dimer

2.721-2.990 [79]

[Au25(PPh3)10(SEt)5Cl2]2+ (39) SbF6 Vertex-sharing icosahedral Au13 dimer

2.695-3.116 [85]

[Au39(PPh3)14Cl6]2+ (40) Cl 1:9:9:1:9:9:1 layered 2.690-3.112 [78]

K. Konishi

Fig. 2 Single-crystal X-ray structures of two isomeric structures of the cluster moieties of [Au9(PPh3)8](PW12O40) (14·PW12O40)

Fig. 3 Single-crystal X-ray structures of Au14 (35) Au20 (36), Au22 (37), Au24 (38), Au25 (40) and Au39 (41) clusters. Counter anion: Cl (36), SbF6 (40), Cl (41) two nearest neighbor nitrated gold atoms are very short (2.58 Å). The icosahedral Au13 and quasi-icosahedral Au11 can serve as the building units of prolate-shaped higher nuclearity clusters when additional attractive forces or steric effects are applied. Q. M. Wang et al. recently reported the structure of [Au20(PPy2Ph)10Cl4]2+ (36), which can be viewed as an edge-sharing dimer of icosahedron-based Au11 cluster [66]. The additional coordination of pyridyl groups to the cluster surface

crown butterfly

crown butterfly

[Au20(PPh(4-Py)2)10Cl4]2+ (36) Au22(dppo)6 (37)Au14(PPh3)8(NO3)4 (35)

[Au24(PPh3)10(SC2H4Ph)5]+ (38) [Au25(PPh3)10(SEt)5]2+ (39) [Au39(PPh3)14Cl6]2+ (40)


appears to assist the holding of the cluster skeleton. Wang et al. reported that Au22(dppo)6 (37), although its charge is ambiguous, has a structure composed of two quasi-icosahedral Au11 units fused via four short Au-Au bridges [73]. There are also examples of the utilization of thiolate as bridging ligands. Shichibu et al. demonstrated that the structure of [Au25(PPh3)10(SC2H5)Cl2]2+ (39) cluster can be defined as two Au13 cluster units joined through five thiolate bridges by vertex sharing [85]. An analogous thiolate-bridged PGC composed of two incomplete icosahedral Au12 units, in which one vertex atom of Au13 icosahedron is missing, has been recently reported by Jin et al. for [Au24(PPh3)10(SC2H4Ph)5X2]+(X = Br or Cl, 38) [79]. The Au-Au bond distances in these icosahedron-based clusters (36–39) are similar to those of smaller centered clusters, being in the range 2.63-3.23 Å (Table 2). Short distances less than 2.7 Å are observed for the center -peripheral radial bonds. On the other hand, the structures of [Au39(PPh3)14Cl6]Cl2 40·Cl) is exceptional, being defined as a 1:9:9:1:9:9:1 layered hcp/hcp', which is virtually different from the icosahedron-based structure. 4.2 Non-centered Polyhedral Clusters Lower nuclearity clusters favor condensed polyhedra rather than centered geometries (Fig. 4). For hexanuclear clusters (N = 6), the simplest geometry is octahedron. Bellon et al. reported an octahedral structure for [Au6{P(p-Tol)3}6]3+ [48], but later it is identified to be a carbon-centered octahedron [100]. Very recently Echavarren et al. reported organometallic Au6 clusters protected by ortho-aurated PPh3 ligands [Au6(PPh2C6H4)4](SbF6)2 (2·SbF6), though their gold atoms are formally monovalent, adopt an octahedron [93]. In this structure, the octahedron is severely distorted, where some edges are short in the range 2.72 - 2.75 Å but the other bonds are longer than 3.10 Å. Au6 cluster can also adopt tetrahedron-based geometries. Mingos et al. reported an edge-sharing bitetrahedral structure of [Au6(PPh3)6]2+ (3) [40], where the shared (2.652 Å) and terminal (2.662 and 2.669 Å) edges are significantly shorter than the other eight edges (2.762 - 2.839 Å) (Table 3). We have recently shown that an edge-sharing Au4 tetrahedron trimer ([Au8(dppp)4]2+, 9), which is an extended version of 3, have a similar structural feature [81]. The bond distances of two shared and terminal edges are 2.623 and 2.607 Å, respectively, which are again shorter than the distances of the remaining connecting bonds (2.824 - 2.896 Å) (Table 3). A face-sharing tritetrahedral structure has been reported for [Au6(xy-xantphos)3]Cl (5), where the edge shared by three Au4 tetrahedra shows a short distance (2.653 Å) [77]. Thus, in the structures based on Au4 tetrahedron, the short distances are found for the shared and terminal edges.

K. Konishi

Table 3 Au-Au bond distances (Å) of Au4 tetrahedron-based clusters

[Au6(PPh3)6]2+ (3)

[Au7(PPh3)7]+ (6) [Au8(dppp)4]2+ (9)

[Au6(xy-xantphos)3]+ (5)

Polyhedral core exo-to-core

References

Distance range(mean)

Shared edge

Terminal edge

Tetrahedral

[Au5(dppm)3(Ph2PCHPPh2)] (NO3)2 (1·NO3)

2.702-3.006 (2.828)

3.013 [23]

[Au6(dppp)4](NO3)2 (4·NO3) 2.630-2.923 (2.825)

- 2.630a 2.798 [26]

[Au8(PMes3)6](BF4)2(10 BF4) 2.697-2.711 (2.707)

- - 2.616, 2.623 [86]

Bitetrahedral

[Au6(PPh3)6](NO3)2 (3 NO3) 2.652-2.839 (2.759)

2.652 2.662, 2.669b

- [40]

[Au8(dppp)4Cl2](PF6)2 (11·PF6) 2.648-2.872 (2.775)

2.648 2.650, 2.658a

2.970-3.072 [81]

[Au8(dppp)4(C!CPh)2](NO3)2

(12·NO3)

2.628-2.865 (2.786)

2.628 2.666a 2.932, 3.019 [91]

[Au8(dppa)4Cl2]Cl2 (13·Cl) 2.619-2.814 (2.728)

2.619 2.621a 2.974-3.052 [96]

Tri-tetrahedral

[Au8(dppp)4](NO3)2 (9 NO3) 2.607-2.896 (2.790)

2.630 2.607b - [81]

[Au6(xy-xantphos)3]Cl (5·Cl) 2.653-3.105 (2.779)

2.653c - [77]

aExo-bridged edges bEdges not containing gold atoms shared by two or more Au4 tetrahedra cThe edge shared by three Au4 tetrahedra

Fig. 4 Single-crystal X-ray structures of non-centered polyhedral Au6, Au7, and Au8 clusters. Counter anion: NO3 (1, 2, 9), Cl (5), C60 (6).


In contrast to the diverse geometries of hexanuclear clusters, pentagonal bipyramid is the only one geometry known for Au7 clusters (6). The distances between the two vertexes are extremely short in the range 2.561-2.581 Å. 4.3 Exo-attached Polyhedral Clusters (core+exo Type) As described in the previous sections, assembling of gold atoms intrinsically favors to form polyhedral skeletons. However, some clusters adopt unusual geometries with extra gold atoms located outside the polyhedral core (“core+exo” type), especially when coupled with bidentate phosphine ligand. The first example reported by van der Verden et al. in 1979 is [Au5(dppm)3(dppm-H)]2+ (1), which has a tetrahedral Au4 core and an additional gold atom outside the tetrahedron [23]. The second example is “blue” Au6 cluster ([Au6(dppp)4]2+, 4) reported by van der Verden et al. in 1982 [26]. The single-crystal structure shows this cluster contains a tetrahedral Au4 core and two gold atoms bridged at opposite edges of the tetrahedron, thus being described as a Au4+2Au type structure. Each of the gold atoms forming the central tetrahedron is coordinated by a single phosphine ligand, while the gold atoms at the exo positions accommodate two phosphine ligands. The core-to-exo distances (distances of exo gold atom from bonded gold atoms in the polyhedral core) are 2.798 Å, which fall in the typical distance range of PGCs (Table 3). For a long time, these have been supposed to be unusual cases, but we have recently reported higher-nuclearity clusters with similar structural features (11, 12, 32 in Fig. 5). [Au8(dppp)4Cl2]2+ (11) has an edge-sharing bi-tetrahedral Au6 core with two exo gold atoms bridged on the terminal edges of the bitetrahedron, thus being viewed as an extended version of 4. The two chloride ligands, each of which is bonded to one of the terminal gold atoms, are forced to be oriented in trans configuration because of the chelation restriction by the dppp ligands. The bitetrahedral Au6 core shows distances of the shared edge of 2.648 Å and the bridged terminal edges of 2.666 Å, which are nearly identical to those of [Au6(PPh3)6]2+ (3) (2.652 and 2.666 Å) and are evidently shorter than the other Au-Au bonds (2.779 – 2.872 Å) (Table 3) [40]. On the other hand, the core-to-exo distances are in the range 2.970 – 3.072 Å, which are markedly longer than Au-Au distance in the bitetrahedral core (2.648 – 2.872 Å). Other Au6+2Au type clusters (12 and 13) were reported recently [91, 96], which show almost similar structural parameters (Table 4). We have also recently synthesized a new dodeca-coordinated Au11 cluster [Au11(dppe)6]3+ (32) (Au11P12 type) [74]. Unlike conventional deca-cooridnated Au11L10 clusters with polyhedral-only geometries (23–31), 32 has a core+exo type geometrical feature. Its polyhedral core is a butterfly-shaped Au9, which resembles the Au9 core of 14a, 15, and 16a. The exo atoms form triangles by sharing an edge of the Au9 core. Each of the exo gold atoms is bonded to the two phosphine ligands, whereas the eight peripheral gold atoms of the Au9 core accommodate single

K. Konishi

Fig. 5 Single-crystal X-ray structures of clusters having a polyhedral core with exo-attached gold atoms. Counter anion: NO3 (4, 12), PF6 (11), BF4 (10), SbF6 (32)

Table 4 Au-Au bond distances (Å) of butterfly-shaped Au9 unit of [Au9(PPh3)8]3+ (14a) and that of Au9+2Au type Au11 cluster (Au11(dppe)6)3+, 32)

Mean dist. of Au9 unit

Radial

Peripheral

exo-to-core

References

[Au9(PPh3)8](NO3)3

(14a·NO3) 2.777 2.675-2.708

(2.691) 2.762-2.880 (2.834) - [62]

[Au11(dppe)6] (SbF6)3

(32·SbF6)

2.814 2.666-2.824 (2.745)

2.629-2.971 (2.860)

2.777, 2.824

[74]

phosphine ligands. The Au9 core unit is apparently similar to previously known examples of butterfly-shaped clusters, although it has a slightly wider range of Au–Au bond distances (2.629–2.971 Å) than [Au9(PPh3)8]3+ (2.675–2.880 Å) (Table 4) [62]. Significantly, the exo-bridged edges are very short (2.629 Å) compared with the shortest peripheral-peripheral bonds of the non-exo type clusters (i.e., 2.762 Å for 14a·NO3). This may reflect some distortion caused by the bridging of the exo gold atoms. The core-to-exo distances were 2.777 and 2.824 Å, which lie in the range of the bond distances in the central butterfly unit.

These core+exo type clusters exhibit impressive colors that are different from conventional centered polyhedral clusters. This aspect will be mentioned in the next section. Among the three core+exo type clusters (Au6, Au8, Au11), Au8 clusters (11–13) are different in the context that they have additional anionic ligands (sub-ligands). The long exo-to-core distance in 11–13 can be attributed to the

[Au6(dppp)4]2+ (4) [Au8(dppp)4Cl2]2+ (11) [Au8(dppp)4(C!CR)2]2+ (12)

[Au11(dppe)6]3+ (32) [Au8(PMes3)6]2+ (10)


electron-withdrawing character of the anionic ligands attached to the exo gold atoms, which may weaken the Au-Au interaction between the exo Au atoms and the central Au6 unit [101]. This is noteworthy in comparison with the short Au-AuCl distances in Au13 clusters [34].

As a different core+exo type geometry, Sharp et al. reported Au8 cluster coordinated by sterically hindered monophosphines ([Au8(PMes3)6]2+ (10) [86]. This cluster has a tetrahedral core with edge distances of 2.697–2.711 Å, where two triangles are attached to the adjacent tetrahedral vertexes. The exo-to-core distances (2.616 and 2.623 Å) are slightly shorter than the tetrahedron edges.

5 Optical Properties

5.1 Electronic Absorption Spectra Generally PGCs are highly colored, so UV-visible electronic absorption spectra have been widely utilized for characterization. Unlike well-known red colors of large gold colloids (sizes of ~ 2 nm or above) offered by localized surface plasmon, the colors of ultrasmall gold clusters including PGCs essentially originate from the discrete electronic transitions, which is supported theoretically [102]. Therefore, the absorption spectra are useful not only for characterization but also to obtain insights into the electronic structures of cluster compounds. On the other hand, in the nano-size regime, it is well accepted that the lowest optical transition energy of metal atom assemblies increases as the size is reduced, which is known as quantum-size effects. In this relation, studies on the relationship between the nuclearity and the absorption spectra would provide useful knowledge about the intermediate between nano-objects and molecules.

5.1.1 Centered Polyhedral Clusters

Solution absorption spectra of Au8P8 (8·BF4), Au9P8 (14·NO3), Au11P8Cl2 (29·Cl) Au11P10 (30·NO3), Au13P10Cl2 (34·Cl) and Au13P10Cl2 (41·Cl) clusters, which adopt centered polyhedral geometries, are shown in Fig. 6a. The spectra except 8·BF4 were measured in our laboratory for crystallographically characterized or equivalently pure samples. The spectrum of 8·BF4 is from the literature [34]. The spectral data together with those in literatures are summarized in Table 5. As a common feature, these centered polyhedral clusters exhibit tail-and-humps spectral characteristics, in which relatively weak visible absorption bands are overlapped with the main band in the UV region tailing to the red. For example, Au9P8 cluster (14) shows the lowest energy visible band at ~ 450 nm and more intense UV band at ~ 315 nm. Au11 clusters ([Au11P8Cl2]+ (29) and [Au11P10]3+ (41)) show sets of a visible band at ~ 420 nm and more intense UV bands at ~300 nm. The spectral

K. Konishi

patterns of the other [Au11P10]3+ clusters (31 and 31) and neutral Au11P7Cl3 cluster (29’) are essentially similar (Table 5) [65, 71], so they seem to be unaffected by the associated surface ligands, cluster charge and counter anions. For Au13 clusters, Mingos et al. have reported that [Au13(PMe2Ph)10Cl2]3+ (33, Au13P10Cl2 type) and [Au13(PMePh2)10Cl4] (33’, Au13P8Cl4 type) clusters give almost identical spectra in DCM with intense UV band at ~340 and a smaller hump (shoulder) at ~430 nm. From this observation, they concluded that the spectra is solely dependent on the nuclearity but is far less so on the surrounding ligands

400 600 700500Wavelength (nm)

Au20 (36)

Au25 (39)

Au24 (38)

Au22 (37)

[Au11P10]3+ (30)

400 600 800Wavelength (nm)

700500300

Abs

[Au13P10Cl2]3+ (34)

[Au8P8]2+ (9)

[Au13P8Cl4]+ (41)

[Au11P8Cl2]+ (29)

[Au11P10]3+ (30)

[Au9P8]3+ (14)

a)

b)

Abs

Fig. 6 Absorption spectra of selected centered clusters at ambient temperature. For solvents and counter ions (see Table 5)


Tab

le 5

El

ectro

nic

abso

rptio

n an

d ph

otol

umin

esce

nce

spec

tral d

ata

of c

rysta

llogr

aphi

cally

cha

ract

eriz

ed c

ente

red

PGCs

and

rela

ted

clus

ters

Pa N

Cl

uste

r for

mul

a So

lven

t A

bs b

ands

(nm

)b !(

M-1 c

m-1)

PL "

em(n

m)c

Refe

renc

es

8 [A

u 8(P

Ph3) 8

](BF 4

) 2 (8

·BF 4

) M

eOH

46

0 41

2 37

0 nd

#

[34]

9

[Au 9

(PPh

3) 8](N

O3) 2

(14·

NO

3) M

eOH

44

3 37

5 35

2 31

4 1.

6 $

104 (4

43 n

m)

NP

[34]

9

[Au 9

(P{p

-MeO

C 6H

4}3) 8

](BF 4

) 3 (1

6·BF

4) D

CM

456

379

342

316

1.

6 $

104 (4

56 n

m)

nd

[34]

11

[A

u 11(P

Ph3) 8

Cl2]C

M

eOH

~5

00 (s

h) 4

21 3

80 3

08

4.0 $

104 (4

21 n

m)

NP

[69]

11

[A

u 11(d

ppp)

5](N

O3) 3

(30·

NO

3) D

CM

~500

(sh)

425

370

301

nd

N

P [6

9]

11

[Au 1

1(PM

ePh 2

) 10](B

Ph4) 3

e(3

1·BP

h 4)

DCM

42

2 37

4, 2

98

2.2 $

104 (4

22 n

m)

nd

[34]

11

[A

u 11(P

MeP

h 2) 10

](CB)

3 (3

1·CB

) D

CM

425

378

302

3.8 $

104 (4

25 n

m)

nd

[42]

11

[A

u 11(P

Me 2

Ph) 10

](BPh

4) 3 e (3

1’·B

Ph4)

DCM

41

2, 3

68, 2

93

3.0 $

104 (4

12nm

) nd

[3

4]

13

[Au 1

3(PM

e 2Ph

) 10Cl

2](PF

6) 3(3

3·PF

6) D

CM

428

338

1.0 $

105 (3

38 n

m)

nd

[34]

13

[A

u 13(P

MeP

h 2) 8C

l 4](C

B) e(3

3’·C

B)

DCM

42

5 (s

h) 3

41

1.2 $

105 (3

41 n

m)

nd

[43]

13

[A

u 13(d

ppe)

5Cl 2]

Cl3 (

34·C

l) M

eCN

49

3 35

9 30

4

9.3 $

104 (3

59 n

m)

766

[75]

13

[A

u 13(d

ppp)

4Cl 4]

Cl (4

1·Cl

) e M

eCN

43

0 34

0 nd

~7

80

[76]

20

[A

u 20(P

Ph{4

-Py}

2) 10C

l 4]Cl

2 (36

·Cl)

DCM

49

3 34

4(sh

) nd

nd

[6

6]

22

Au 2

2(dpp

o)6 (

37)

DCM

45

6 nd

~7

00

[73]

24

[A

u 24(P

Ph3) 1

0(SC 2

H4P

h)5X

2]+ (38)

D

CM

560

383

nd

~818

f [7

9]

25

[Au 2

5(PPh

3) 10(S

Et) 5C

l 2]2+

(39)

D

CM

670,

415

nd

~1

000

[85]

a N

P: n

o ph

otol

umin

esce

nce;

nd:

no

data

giv

en

b Show

n fo

r the

ban

ds a

t > 2

90 n

m

c Exci

tatio

n at

low

est-e

nerg

y ab

sorp

tion

band

d CB

: C2B

9H12

e N

ot id

entif

ied

by si

ngle

cry

stal X

-ray

diffr

actio

n bu

t the

pur

ity w

as c

heck

ed b

y m

ass s

pect

ral a

nd/o

r ele

men

tal a

naly

ses

f "ex

= 5

00 n

m

K. Konishi

(P:Cl ratio). We observed similar spectral patterns for [Au13(dppp)4Cl4]+ (41, Au13P8Cl4 type) in MeCN (Fig. 6a) and in DCM [76]. On the other hand, the spectrum of [Au13(dppe)5Cl2]3+ (34, Au13P10Cl2 type) in MeCN is markedly different, showing a main band at ~360 nm together with a broad weaker band at ~500 nm (Fig. 6a) [75]. Therefore, for the diphosphine-coordinated series, the absorption profiles are critically dependent on the number of Cl ligand ratio, suggesting the involvement of the ligand-mediated transitions in the observed electronic absorptions.

The spectra of higher-nuclearity centered clusters (36–39) taken from the literatures [66, 73, 79, 85] are summarized in Fig. 6b. They all exhibit tail-and-humps profiles. There seems no relationship between the nuclearity and the positions of the lowest-energy visible bands, and the transitions are interpreted in individual cases. For 36 and 37, whose structures can be viewed as the dimers of the icosahedron- based Au11 clusters, the visible bands are observed at lower energies than that of the parent cluster. For 39, the exceptionally low-energy band (680 nm) is noteworthy, which suggests the effective electronic interaction between two Au13 cluster units via vertex sharing.

5.1.2 Non-centered and Core+exo Clusters

Figure 7 shows the solution absorption spectra of edge-sharing bitetrahedral Au6P6 cluster (3·NO3), tritetrahedral Au8P6 cluster (9·NO3), and [core+exo]-type Au6P8 (4·NO3) Au8P8Cl2 (11·Cl), and Au11P12 (32·SbF6) clusters, together with the spectra of centered toroidal Au9P8 (14·NO3) and spherical Au11P10 (30·NO3) clusters for comparison. Except the spectrum of 6, which is taken from the literature [34], all spectra were measured in our laboratory and the sample purity was checked by ESI-MS and NMR. The spectral data are summarized in Table 6. Similarly to the above-mentioned cases of centered polyhedral core-only clusters, the spectrum of the bitetrahedral cluster (3) can be described as the combination of intense UV bands and relatively weak visible bands. However, the visible bands at 452 and 476 nm are considerably separated from the UV bands. A similar trend was observed for the edge-sharing trimer of the tetrahedral Au4 motif (9). The spectrum showed a band at 520 nm together with a smaller shoulder band at ~590 nm, which are completely separated from the UV band at 308 nm. At the present stage, these transitions are not assigned, but seem to be a common feature of edge-sharing tetrahedral clusters. The low energies of the visible absorptions of the trimer (9) when compared with the dimer (3) can be attributed to the simple mixing of MOs of the tetrahedral-based units (bitetrahedron and tetrahedron). [Core+exo]-type clusters having edge-sharing gold triangle moieties exhibit more distinctive spectral features. For example, Au4+2Au type Au6P8 (4), Au6+2Au type Au8P8Cl2 (11), and Au9+2Au type Au11P12 (32) all showed single visible bands at 587, 508 and 663 nm, respectively, which are fairly separated from the short-wavelength bands spreading into the UV regions. These spectra are much different in pattern from the tail-and-humps feature of centered only-polyhedral


Fig. 7 Solution absorption spectra of selected non-centered and core+exo clusters at ambient temperature. For conditions and counter ions (see Table 5) clusters such as toroidal Au9P8 cluster (14) and spherical Au11P8 cluster (30). Therefore, the “isolated” absorption bands are likely a common feature of [core +exo] type clusters. The attachment of extra gold atoms to the polyhedral core may allow the generation of new electronic structures that are associated with the characteristic visible absorptions. Actually the absorption bands of 32 did not coincide with any bands of [Au9(PPh3)8]3+ (14), which has a toroidal core and thus can be a model of the central substructure of 32. It should be also noted that the lowest transition energy increased in the order Au11 (32, 1.87 eV) < Au6 (4, 2.11 eV) < Au8 (11, 2.44 eV), not matching the order of nuclearity. Furthermore, noticeable differences in the spectral profiles are found between the isomeric structures of Au6 (3 vs 4), Au8 (8 vs 9 vs 11) and Au11 (30 vs 32). Therefore, the optical properties of these clusters depend more strongly on geometric structure than on nuclearity. Table 6 summarizes the spectra data of the tetrahedron-based and core+exo type clusters. For the core+exo series, the ! values of the all-phosphine type clusters (3 and 32) are noticeably larger than those of 8 and 9 with anionic ligands, which may be correlated with the long core-to-exo distances (Tables 3 and 4). Density functional theory (DFT) studies on corresponding non-phenyl models of 3, 8 and 30 show that they have similar electronic structures with discrete HOMO and LUMO bands, which are predominantly constituted by Au(6sp) orbitals (sp bands) and are appreciably separated from the closest orbitals. The theoretical

TrTh-Au8 (9)

Au6+2AuCl (11)

Au4+2Au (4)

Au9+2Au (32)

BiTh-Au6 (3)

[Au11P10]3+ (30)

[Au9P8]3+ (14)

K. Konishi

Tab

le 6

Ele

ctro

nic

abso

rptio

n sp

ectra

l dat

a of

non

-cen

tere

d an

d co

re+e

xo ty

pe P

GCs

a, b

N

Clus

ter f

orm

ula

Stru

ctur

e ty

pe

Abs

ban

ds(n

m)

!(M

-1 c

m-1)

PL "

em (n

m)d

Refe

renc

es

6 [A

u 6(P

Ph3) 6

](NO

3) 2 (3

·NO

3) Bi

tetra

hedr

al

476

452

330

319

nd

nd

[34]

8

[Au 8

(dpp

p)4](

NO

3) 2 (9

·NO

3) Tr

itetra

hedr

al

590

(sh)

520

308

3.

0 #

104 (5

20 n

m)

NL

[81]

6

[Au 6

(dpp

p)4](

NO

3) 2 (4

·NO

3) A

u 4+2

Au

587

432

326

c 8.

9 #

104 (5

87 n

m)

nde

[101

] 8

[Au 8

(dpp

p)4C

l 2]Cl

2 (1

1·Cl

) A

u 6+2

Au

508

390

322

2.7 #

104 (5

08 n

m)

597

[81]

8

[Au 8

(dpp

p)4(C

!CPh

) 2](N

O3) 2

(12·

NO

3) A

u 6+2

Au

509

393

325

3.9 #

104 (5

09 n

m)

577

[91]

11

[A

u 11(d

ppe)

6](Sb

F 6) 3

(32·

SbF 6

) A

u 9+2

Au

663

471

390

316

8.9 #

104 (6

63 n

m)

nde

[74]

a N

L no

n-lu

min

esce

nt, n

d no

dat

a gi

ven

b In D

CM a

t roo

m te

mpe

ratu

re u

nles

s oth

erw

ise n

oted

. Alm

ost i

dent

ical

spec

tra w

ere

obta

ined

in M

eCN

and

MeO

H

c In M

eOH

d Ex

cite

d at

the

low

est-e

nerg

y ab

sorp

tion

band

e Es

sent

ially

PL

activ

e (F

ig. 9

b)


spectra generated by time-dependent DFT (TD-DFT) calculations reproduce well the experimental spectra, leading to the assignment of the characteristic visible bands to intermetal HOMO-LUMO transitions. Orbital distribution analyses demonstrate the HOMO and LUMO are both found in proximity to the terminal Au3 triangles containing the exo gold atom, and the HOMO ! LUMO transition occurs in the core ! exo direction (Fig. 8).

5.1.3 Summary

Electronic absorption properties of PGCs are strictly dependent not only on the nuclearity but also the geometrical structures of the cluster units. Centered polyhedral-only clusters generally show tail-and-hump spectra, while tetrahedron-based and [core+exo]-type clusters give isolated absorption bands in the visible region. Except some exceptions, the spectral profiles are essentially defined by the nuclearity/geometry of the gold cluster units, and the modification of the surface ligands cause little effects on the spectral pattern. The transition energies (band positions) show no clear correlation with the order of nuclearity and vary significantly with the individual arrangements of gold atoms, indicating the strong “molecular” characters of this class of cluster compounds. 5.2 Photoluminescence In recent years there has been intense interest in the luminescence properties of ultrasmall noble metal clusters due to their potential utility as quantum dots [13]. However, the origin of the luminescence has not been elucidated yet because of the lack of the geometrical information. Ligand-coordinated ultrasmall gold clusters including PGCs are potentially photoluminescent due to the small size, but there have been very limited studies. We found that [Au9(PPh3)8]3+ (14) and [Au11L10]3+ clusters (29, 29’ and 30) are virtually PL inactive. In contrast, Au13P10Cl2 cluster 34 is appreciably photoluminescent, giving a PL band in the near IR region (766 nm) when excited at 500 nm, with a moderately high quantum

Fig. 8 Schematic illustration of HOMO and LUMO of [Au11(dppe)6]3+ (32)

K. Konishi

Fig. 9 Absorption (dotted lines) and PL (solid lines) spectra of two isomeric forms of Au8 clusters: 9·NO3 (red) and 11·Cl (black) in MeOH at 25°C.

Fig. 10 Pictures of solution samples of PGCs (a) under room light and (b) under irradiation of a 365-nm handy UV lamp. (Left to right) [Au6(dppp)4](NO3)2 (4·NO3); [Au8(dppp)4](NO3)2 (9·NO3); [Au8(dppp)4Cl2](Cl)2 (11·Cl); [Au9(PPh3)8](NO3)2 (14·NO3); [Au11(PPh3)8Cl2]Cl (29·Cl); [Au11(dppe)6](SbF6)3 (32·SbF6); [Au13(dppe)5Cl2]Cl3 (34·Cl) yield (0.076) [75]. On the other hand, the Au13P8Cl4 cluster 41 shows a PL band at a similar wavelength but the quantum efficiency is considerably small (0.008), indicating that the local environment (ligand) in proximity to the gold core notably affects the radiative path that contributes the PL emission. We have also found that [core+exo]-type Au6, Au8 and Au11 clusters are essentially PL active (Fig. 10b). For example, [Au8(dppp)4Cl2]2+ (11) shows an evident photoluminescence band at 597 nm upon excitation at 509 nm (Fig. 9, black solid line). The excitation spectrum monitored at 600 nm almost coincided with the absorption spectrum. Similarly to the above-mentioned Au13 clusters, the PL profiles

9[Au8(L)4]2+

11[Au8(L)4Cl2]2+

PL

9

400 600Wavelength (nm)

700500

Abs

11


of the core+exo type Au8 clusters appear to be notably affected by the sub-ligand (e.g., Cl in 11). [Au8(dppp)4(C!CPh)2](NO3)2 (12·NO3), which has two acetylide units in place of chloride ligands, shows a similar PL band at a noticeably higher energy (577 nm). On the other hand, [Au8(dppp)4](NO3)2 (9·NO3) having a tetrahedron trimer motif, which is isomeric to that of 11 and 12, was almost PL inactive (red solid line), again indicating that the optical properties of PGCs are strictly dependent on their geometries. 5.3 Gold Clusters as Stimuli-Responsive Chromic Modules As expected from the visible absorption profiles mentioned above, PGCs exhibit a wide variety of colors ranging over the whole visible region. Fig. 10 shows the pictures of the solution samples of representative clusters under room light and UV-light, which reveal that their colors and PL activities are highly dependent on the number (nuclearity) and arrangement (geometry) of the gold atoms. Especially, the colors of the core+exo type clusters are impressive; their blue, pink, and green colors are unusual for gold clusters, which are markedly different from the plasmon-like reddish colors of conventional centered clusters such as Au9, Au11 and Au13. This is surprising in the sense that the components of the clusters are virtually same and the optical property is simply governed by the arrangement of the gold atoms. The subtle difference in the arrangement leads to the great color differences.

In this context, one of the prominent features of gold clusters is their soft potential energy surfaces. This potentially allows the reversible interconversion among several particular geometries, through which the optical properties of the clusters would be altered. This idea can be supported by the earlier work by Mingos et al., who provide an example of the color and reflectance spectral difference between the butterfly and crown isomers of Au9 cluster in the solid state [38]. Thus, if the cluster geometry can be tuned by external stimuli, one would see the optical responses in color change or PL on-off. Therefore, these gold clusters have a unique potential to serve as stimuli-responsive chromism modules and may be applied to chemical sensors. As an example, we have recently shown a redox-induced chromism system based on the isomerization of the Au8 units between 9 and 11 (Fig. 11) [81]. Thus, the formal charges of the Au8 units of 9 and 11 were 2+ and 4+, respectively, which can be reversibly converted into each other via two-electron redox processes. As noted in the previous sections, the reduced form (9) is violet in color and PL inactive, whereas the oxidized form (11) is pink in color and PL active. As shown in Fig. 10, it is possible to switch the optical properties reversibly (color and PL) through chemical reduction/oxidation. The use of electrochemical processes would be also feasible, so the present cluster system has the potential for redox-based functions such as electrochromism.

K. Konishi

Fig. 11 Redox-mediated chromism between 9 and 11 associated with the isomerization of the Au8 cluster framework

Fig. 12 Protonation-induced chromism of 4-pyridylethynyl-appended Au8 cluster

Based on the general trend of the optical properties mentioned above, it is obvious that the nuclearity and geometries are primary factors to govern the optical properties of molecular gold clusters. In addition, the alteration of nearby or external environments or physical stimuli could affect the optical properties, but such aspects have not been explored. We have recently found that the electronic perturbation coupled with $-conjugated system actually affects the optical properties of the cluster units [91]. As shown in Fig. 12, the [core+exo]-type Au8 clusters bearing two pyridylethynyl ligands on the exo gold atoms ([Au8(dppp)4(C!CPy)2]2+) exhibit reversible visible absorption and photoluminescence responses to protonation/deprotonation events of the terminal pyridyl moieties. It is demonstrated that the chromism behaviors are highly dependent on the relative position of the pyridine nitrogen atom. Therefore the formation of extended charged resonance structures upon pyridine protonation causes significant perturbation effects on the electronic properties of the Au8 unit. This is an example in which organic $-conjugated system and inorganic gold cluster are electronically combined, providing an implication for the design of cluster-based stimuli-responsive chromic modules.

9[Au8(L)4]2+

11[Au8(L)4Cl2]2+

(O2 / X–)oxidation

reduction(NaBH4)

([Au8]4+)([Au8]2+)

pink

PL activePL inactive

violet

protonated

H+

baseResonance-coupled cation transfer

[Au8L4(C!C(4-Py))2]2+

!"#$#%&'

()#*+,#*%&

free


6 Concluding Remarks In the present chapter, the syntheses, X-ray structures (geometries), and optical properties of phosphine-coordinated molecular gold clusters (PGCs) have been summarized. The inspection of the optical properties in the context of the geometrical structures reveals a strong molecular character of PGCs. In addition to the conventional clusters discovered more than 30 years ago, several new families with unique geometrical and optical features have now been identified, thereby expanding the scope of this class of compounds. Among them, unusual optical properties of a series of [core+exo]-type clusters are quite interesting, which demonstrates that the attachment of only one or two gold atoms to polyhedral cores dramatically changes the electronic structures of total metal entities. This principle would be applicable to the colloidal system, which will benefit the rational design of functional colloids. It has been demonstrated that the use of bidentate ligands occasionally allows the generation of unusual cluster species, but it is still at a phenomenological level. The computer-aided design of multidentate phosphines would promote the emergence of unique PGCs. Finally, as shown in the last topic, the electronic structures (optical properties) of PGCs can be altered not only by the geometrical parameters but also by the electronic perturbation by the external stimuli. The combination with appropriate organic units as well as the design/discovery of novel skeletons would lead to the evolution of PGCs to functional materials.

Acknowledgements The author is grateful to Professor Michael Mingos for reading the drafts of this chapter and providing useful comments, and to Dr. Yutaro Kamei for his help in preparing graphics and also checking the structural and spectral data. References 1. Daniel M-C, Astruc D (2004) Gold nanoparticles: assembly, supramolecular chemistry,

quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology. Chem Rev 104 (1):293-346

2. Fernandez EJ, Monge M (2008) Gold nanomaterials. In: Laguna A (ed) Modern Supramolecular Gold Chemistry: Gold-Metal Interactions and Applications. WILEY-VCH Weinheim, pp 131-179. doi:10.1002/9783527623778.ch3

3. Sardar R, Funston A, Mulvaney P, Murray R (2009) Gold nanoparticles: past, present, and future. Langmuir 25 (24):13840-13851

4. Giljohann D, Seferos D, Daniel W, Massich M, Patel P, Mirkin C (2010) Gold nanoparticles for biology and medicine. Angew Chem Int Ed 49 (19):3280-3294

5. Takei T, Akita T, Nakamura I, Fujitani T, Okumura M, Okazaki K, Huang J, Ishida T, Haruta M (2012) Heterogeneous Catalysis by Gold. Adv Catal 55:1-126

6. Hutchings GJ, Edwards JK (2012) Application of Gold Nanoparticles in Catalysis. Metal Nanoparticles and Nanoalloys”, eds RL Johnstone and JP Wilcoxon, Frontiers of Nanoscience, Elsevier Ltd, Oxford, UK 3:249-293

K. Konishi

7. Wilson R (2008) The use of gold nanoparticles in diagnostics and detection. Chem Soc Rev 37 (9):2028-2045

8. Bai C, Liu M (2013) From chemistry to nanoscience: not just a matter of size. Angew Chem Int Ed 52 (10):2678-2683

9. Lu Y, Chen W (2012) Sub-nanometre sized metal clusters: from synthetic challenges to the unique property discoveries. Chem Soc Rev 41 (9):3594-3623

10. Tsukuda T (2012) Toward an Atomic-Level Understanding of Size-Specific Properties of Protected and Stabilized Gold Clusters. Bull Chem Soc Jpn 85 (2):151-168

11. Jin R (2010) Quantum sized, thiolate-protected gold nanoclusters. Nanoscale 2 (3):343-362 12. Häkkinen H (2012) The gold-sulfur interface at the nanoscale. Nature Chem 4 (6):443-455 13. Zheng J, Nicovich P, Dickson R (2007) Highly fluorescent noble-metal quantum dots. Ann

Rev Phys Chem 58:409-431 14. Jadzinsky P, Calero G, Ackerson C, Bushnell D, Kornberg R (2007) Structure of a thiol

monolayer-protected gold nanoparticle at 1.1 A resolution. Science 318 (5849):430-433 15. Heaven MW, Dass A, White PS, Holt KM, Murray RW (2008) Crystal structure of the

gold nanoparticle [N(C8H17)4][Au25(SCH2CH2Ph)18]. J Am Chem Soc 130 (12):3754-3755 16. Zhu M, Eckenhoff WT, Pintauer T, Jin R (2008) Conversion of Anionic

[Au25(SCH2CH2Ph)18]%Cluster to Charge Neutral Cluster via Air Oxidation. The Journal of Physical Chemistry C 112 (37):14221-14224

17. Zhu M, Aikens C, Hollander F, Schatz G, Jin R (2008) Correlating the crystal structure of a thiol-protected Au25 cluster and optical properties. J Am Chem Soc 130 (18):5883-5885.

18. Qian H, Eckenhoff W, Zhu Y, Pintauer T, Jin R (2010) Total structure determination of thiolate-protected Au38 nanoparticles. J Am Chem Soc 132 (24):8280-8281.

19. Naldini L, Cariati F, Simonetta G, Malatesta L (1966) Gold?tertiary phosphine derivatives with intermetallic bonds. Chem Commun (18):647

20. Vollenbroek FA, Bosman WP, Bour JJ, Noordik JH, Beurskens PT (1979) Reactions of gold-phosphine cluster compounds. Preparation and X-ray structure determination of octakis(triphenylphosphine)octa-gold bis(hexafluorophosphate). J Chem Soc, Chem Commun (9):387-388

21. Vollenbroek FA, Bour JJ, van der Veden JWA (1980) Gold-phosphine cluster compounds: The reactions of [Au9L8]3+ (L = PPh3) with L, SCN% and Cl% to [Au8L8]2+ (Au11L8(SCN)2)+ and [Au11L8Cl2]+. Recl Trav Chim Pays-Bas 99 (4):137-141

22. van der Velden JWA, Bour JJ, Bosman WP, Noordik JH (1981) Synthesis and X-ray crystal structure determination of the cationic gold cluster compound [Au8(PPh3)7](NO3)2. J Chem Soc, Chem Commun (23):1218-1219

23. van der Velden JWA, Vollenbroek FA, Bour JJ, Beurskens PT, Smits JMM, Bosnian WP (1981) Gold clusters containing bidentate phosphine ligands. Preparation and X-Ray structure investigation of [Au5(dppmH)3(dppm)](NO3)2 and [Au13(dppmH)6](NO3)n. Recl Trav Chim Pays-Bas 100 (4):148-152

24. Vollenbroek FA, van der Velden JWA, Bour JJ, Trooster JM (1981) Mössbauer investigation of gold cluster compounds. Recl Trav Chim Pays-Bas 100 (10):375-377

25. Steggerda JJ, Bour JJ, Velden JWAvd (1982) Preparation and properties of gold cluster compounds. Recl Trav Chim Pays-Bas 101:164-170

26. Van der Velden JWA, Bour JJ, Steggerda JJ, Beurskens PT, Roseboom M, Noordik JH (1982) Gold clusters. Tetrakis[1,3-bis(diphenylphosphino)propane]hexagold dinitrate: preparation, x-ray analysis, and gold-197 Moessbauer and phosphorus-31{proton} NMR spectra. Inorg Chem 21 (12):4321-4324

27. Smits JMM, Beurskens PT, Bour JJ, Vollenbroek FA (1983) X-ray analysis of octakis(tri-p-tolylphosphine) enneagoldtris (hexafluorophosphate), [Au9{P(p-Me C6H4)3}8](PF6)3 : A redetermination. J Cryst Spect Res 13 (5):365-372

28. Smits JMM, Beurskens PT, Velden JWA, Bour JJ (1983) Partial X-ray analysis of triiodo-heptakis (triphenylphosphine)undecagold, Au11C126H105I3P7. J Cryst Spect Res 13(5): 373-379


29. Smits JMM, Bour JJ, Vollenbroek FA, Beurskens PT (1983) Preparation and X-ray structure determination of [pentakis{1,3-bis(diphenylphosphino)propane}] undecagoldtris(thiocyanate), [Au11{PPh2C3H6PPh2}5](SCN)3. J Cryst Spect Res 13 (5):355-363

30. Van der Velden JWA, Bour JJ, Bosman WP, Noordik JH (1983) Reactions of cationic gold clusters with Lewis bases. Preparation and x-ray structure investigation of [Au8(PPh3)7](NO3)2.2CH2Cl2 and Au6(PPh3)4[Co(CO)4]2. Inorg Chem 22 (13):1913-1918

31. Van der Velden JWA, Beurskens PT, Bour JJ, Bosman WP, Noordik JH, Kolenbrander M, Buskes JAKM (1984) Intermediates in the formation of gold clusters. Preparation and x-ray analysis of [Au7(PPh3)7]+ and synthesis and characterization of [Au8(PPh3)6I]PF6. Inorg Chem 23 (2):146-151

32. van der Velden JWA, Bour JJ, Bosman WP, Noordik JH, Beurskens PT (1984) The electrochemical preparation of [Au9(PPh3)8]+. A comparative study of the structures and properties of [Au9(PPh3)8]+ and [Au9(PPh3)8]3+. Recl Trav Chim Pays-Bas 103 (1):13-16

33. Bos W, Kanters RPF, Van Halen CJ, Bosman WP, Behm H, Smits JMM, Beurskens PT, Bour JJ, Pignolet LH (1986) Gold clusters: synthesis and characterization of [Au8(PPh3)7(CNR)]2+, [Au9(PPh3)6(CNR)2]3+ and [Au11(PPH3)7(CNR)2I]2+ and their reactivity towards amines. The crystal structure of [Au11(PPh3)7(CN-i-Pr)2I](PF6)2. J Organomet Chem 307 (3):385-398

34. Hall KP, Mingos DMP (1984) Homo-and heteronuclear cluster compounds of gold. Prog Inorg Chem 32:237-325

35. Mingos DMP (1984) Gold cluster compounds. Gold Bull 17 (1):5-12 36. Briant CE, Theobald BRC, White JW, Bell LK, Mingos DMP, Welch AJ (1981) Synthesis

and X-ray structural characterization of the centred icosahedral gold cluster compound [Au13(PMe2Ph)10Cl2](PF6)3; the realization of a theoretical prediction. J Chem Soc, Chem Commun (5):201

37. Hall KP, Theoblad BRC, Gilmour DI, Mingos DMP, Welch AJ (1982) Synthesis and structural characterization of [Au9{P(p-C6H4OMe)3}8](BF4)3; a cluster with a centred crown of gold atoms. J Chem Soc, Chem Commun (10):528-530

38. Briant CE, Hall KP, Mingos DMP (1984) Structural characterisation of two crystalline modifications of [Au9{P(C6H4OMe-p)3}8](NO3)3: the first example of skeletal isomerism in metal cluster chemistry. J Chem Soc, Chem Commun (5):290-292

39. Briant CE, Hall KP, Wheeler AC, Mingos DMP (1984) Structural characterisation of [Au10Cl3(PCy2Ph)6](NO3)(Cy = cyclohexyl) and the development of a structural principle for high nuclearity gold clusters. J Chem Soc, Chem Commun (4):248-250

40. Briant CE, Hall KP, Mingos DMP, Wheeler AC (1986) Synthesis and structural characterisation of hexakis(triphenyl phosphine)hexagold(2+) nitrate, [Au6(PPh3)6][NO3]2, and related clusters with edgesharing bitetrahedral geometries. J Chem Soc, Dalton Trans (3):687-692

41. Cheetham GMT, Harding MM, Haggitt JL, Mingos DMP, Powell HR (1993) Synthesis and microcrystal structure determination of [Au10(PPh3)7{S2C2(CN)2}2] with monochromatic synchrotron radiation. J Chem Soc, Chem Commun (12):1000-1001

42. Copley RCB, Mingos DMP (1996) The novel structure of the [Au11(PMePh2)10]3+ cation: crystal structures of [Au11(PMePh2)10][C2B9H12]3·4thf and [Au11(PMePh2)10][C2B9H12]3(thf = tetrahydrofuran). J Chem Soc, Dalton Trans (4):479-489

43. Copley RCB, Mingos DMP (1996) Synthesis and characterization of the centred icosahedral cluster series [Au9MIB4Cl4(PMePh2)8][C2B9H12], where MIB= Au, Ag or Cu. J Chem Soc, Dalton Trans (4):491

44. Cariati F, Naldini L, Simonetta G, Malatesta L (1967) Clusters of gold compounds with 1,2Bis(diphenylphosphino)ethane. Inorg Chim Acta 1:315-318

45. Albano VG, Bellon PL, Manasser M, Sansoni M (1970) Intermetallic Pattern in Metal-Atom Clusters - Structural Studies on Au11X3(PR3)7 Species. Journal of the Chemical Society D-Chemical Communications (18):1210-1211

K. Konishi

46. Cariati F, Naldini L (1971) Trianionoeptakis(triarylphosphine)undecagold cluster compounds. Inorg Chim Acta 5:172-174

47. Bellon P, Manassero M, Sansoni M (1972) Crystal and molecular structure of tri-iodoheptakis(tri-p-fluorophenylphosphine)undecagold. J Chem Soc, Dalton Trans (14):1481-1487

48. Bellon P, Manassero M, Sansoni M (1973) An octahedral gold cluster: crystal and molecular structure of hexakis[tris-(p-tolyl)phosphine]-octahedro-hexagold bis(tetraphenylborate). J Chem Soc, Dalton Trans (22):2423-2427

49. Manassero M, Naldini L, Sansoni M (1979) A new class of gold cluster compounds. Synthesis and X-ray structure of the octakis(triphenylphosphinegold) dializarinsulphonate, [Au8(PPh3)8](aliz)2. J Chem Soc, Chem Commun (9):385-386

50. Mingos DMP (1983) Polyhedral skeletal electron pair approach. A generalised principle for condensed polyhedra. J Chem Soc, Chem Commun (12):706

51. Mingos DMP, Slee T, Zhenyang L (1990) Bonding models for ligated and bare clusters. Chem Rev 90 (2):383-402

52. Wales DJ (2005) Electronic Structure of Clusters. Encyclopedia of Inorganic Chemistry. doi:10.1002/9781119951438.eibc0066

53. Schwerdtfeger P (2003) Gold goes nano--from small clusters to low-dimensional assemblies. Angew Chem Int Ed 42 (17):1892-1895

54. Schmid G (2008) Ionically cross-linked gold clusters and gold nanoparticles. Angew Chem Int Ed 47 (19):3496-3498

55. Schmid G (2008) The relevance of shape and size of Au55 clusters. Chem Soc Rev 37 (9):1909-1930

56. Yam V, Cheng E (2008) Highlights on the recent advances in gold chemistry--a photophysical perspective. Chem Soc Rev 37 (9):1806-1813

57. Koshevoy I, Chang Y-C, Karttunen A, Selivanov S, Jänis J, Haukka M, Pakkanen T, Tunik S, Chou P-T (2012) Intensely luminescent homoleptic alkynyl decanuclear gold(I) clusters and their cationic octanuclear phosphine derivatives. Inorg Chem 51 (13):7392-7403

58. Mingos DMP, Watson MJ (1992) Heteronuclear gold cluster compounds. Adv Inorg Chem 39:327-399

59. Teo BK, Zhang H (1995) Polyicosahedricity: icosahedron to icosahedron of icosahedra growth pathway for bimetallic (Au–Ag) and trimetallic (Au–Ag–M; M = Pt, Pd, Ni) supraclusters; synthetic strategies, site preference, and stereochemical principles. Coord Chem Rev 143:611-636

60. Schmidbaur H, Schier A (2012) Aurophilic interactions as a subject of current research: an up-date. Chem Soc Rev 41 (1):370-412

61. Schulz-Dobrick M, Jansen M (2006) Supramolecular Intercluster Compounds Consisting of Gold Clusters and Keggin Anions. Eur J Inorg Chem 2006 (22):4498-4502

62. Wen F, Englert U, Gutrath B, Simon U (2008) Crystal Structure, Electrochemical and Optical Properties of [Au9(PPh3)8](NO3)3. Eur J Inorg Chem 2008 (1):106-111

63. Cariati F, Naldini L, Simonetta G, Malatesta L (1967) Ethyldiphenylphosphine-gold derivatives with intermetallic bonds. Inorg Chim Acta 1:24-26

64. Bartlett PA, Bauer B, Singer SJ (1978) Synthesis of water-soluble undecagold cluster compounds of potential importance in electron microscopic and other studies of biological systems. J Am Chem Soc 100 (16):5085-5089

65. Gutrath BS, Englert U, Wang Y, Simon U (2013) A Missing Link in Undecagold Cluster Chemistry: Single-Crystal X-ray Analysis of [Au11(PPh3)7Cl3]. Eur J Inorg Chem 2013 (12):2002-2006

66. Wan X-K, Lin Z-W, Wang Q-M (2012) Au20 nanocluster protected by hemilabile phosphines. J Am Chem Soc 134 (36):14750-14752

67. Schmid G, Pfeil R, Boese R, Bandermann F, Meyer S, Calis GHM, van der Velden JWA (1981) Au55[P(C6H5)3]12CI6 — ein Goldcluster ungewöhnlicher Größe. Chem Ber 114 (11):3634-3642


68. Bertino M, Sun Z-M, Zhang R, Wang L-S (2006) Facile syntheses of monodisperse ultrasmall Au clusters. J Phys Chem B 110 (43):21416-21418

69. Kamei Y, Shichibu Y, Konishi K. unpublished results 70. Golightly JS, Gao L, Castleman AW, Bergeron DE, Hudgens JW, Magyar RJ, Gonzalez CA

(2007) Impact of Swapping Ethyl for Phenyl Groups on Diphosphine-Protected Undecagold. J Phys Chem C 111

71. Yanagimoto Y, Negishi Y, Fujihara H, Tsukuda T (2006) Chiroptical activity of BINAP-stabilized undecagold clusters. J Phys Chem B 110 (24):11611-11614

72. Andreiadis E, Vitale M, Mézailles N, Le Goff X, Le Floch P, Toullec P, Michelet V (2010) Chiral undecagold clusters: synthesis, characterization and investigation in catalysis. Dalton Trans 39 (44):10608-10616

73. Chen J, Zhang QF, Bonaccorso TA, Williard PG, Wang LS (2014) Controlling gold nanoclusters by diphospine ligands. J Am Chem Soc 136 (1):92-95

74. Shichibu Y, Kamei Y, Konishi K (2012) Unique [core+two] structure and optical property of a dodeca-ligated undecagold cluster: critical contribution of the exo gold atoms to the electronic structure. Chem Commun 48 (61):7559-7561

75. Shichibu Y, Konishi K (2010) HCl-induced nuclearity convergence in diphosphine-protected ultrasmall gold clusters: a novel synthetic route to "magic-number" Au13 clusters. Small 6 (11):1216-1220

76. Shichibu Y, Suzuki K, Konishi K (2012) Facile synthesis and optical properties of magic-number Au13 clusters. Nanoscale 4 (14):4125-4129

77. Ito H, Saito T, Miyahara T, Zhong C, Sawamura M (2009) Gold(I) Hydride Intermediate in Catalysis: Dehydrogenative Alcohol Silylation Catalyzed by Gold(I) Complex. Organometallics 28 (16):4829-4840

78. Teo BK, Shi X, Zhang H (1992) Pure gold cluster of 1:9:9:1:9:9:1 layered structure: a novel 39-metal-atom cluster [(Ph3P)14Au39Cl6]Cl2 with an interstitial gold atom in a hexagonal antiprismatic cage. J Am Chem Soc 114 (7):2743-2745

79. Das A, Li T, Nobusada K, Zeng Q, Rosi N, Jin R (2012) Total structure and optical properties of a phosphine/thiolate-protected Au24 nanocluster. J Am Chem Soc 134 (50):20286-20289.

80. Strähle J (1995) Synthesis of cluster compounds by photolysis of azido complexes. J Organomet Chem 488 (1-2):15-24

81. Kamei Y, Shichibu Y, Konishi K (2011) Generation of small gold clusters with unique geometries through cluster-to-cluster transformations: octanuclear clusters with edge-sharing gold tetrahedron motifs. Angew Chem Int Ed 50 (32):7442-7445

82. Laguna A, Laguna M, Gimeno MC, Jones PG (1992) Synthesis and x-ray characterization of the neutral organometallic gold cluster [Au10(C6F5)4(PPh3)5]. Organometallics 11 (8):2759-2760

83. Vollenbroek FA, Bour JJ, Trooster JM, van der Velden JWA (1978) Reactions of gold-phosphine cluster compounds. J Chem Soc, Chem Commun (21):907

84. Nunokawa K, Onaka S, Ito M, Horibe M, Yonezawa T, Nishihara H, Ozeki T, Chiba H, Watase S, Nakamoto M (2006) Synthesis, single crystal X-ray analysis, and TEM for a single-sized Au11 cluster stabilized by SR ligands: The interface between molecules and particles. J Organomet Chem 691 (4):638-642

85. Shichibu Y, Negishi Y, Watanabe T, Chaki NK, Kawaguchi H, Tsukuda T (2007) Biicosahedral Gold Clusters [Au25(PPh3)10(SCnH2n+1)5Cl2]2+ (n = 2-18): A Stepping Stone to Cluster-Assembled Materials. J Phys Chem C 111 (22):7845-7847

86. Yang Y, Sharp PR (1994) New Gold Clusters [Au8L6](BF4)2 and [(AuL)4](BF4)2 (L = P(mesityl)3). J Am Chem Soc 116 (15):6983-6984

87. Ramamoorthy V, Wu Z, Yi Y, Sharp PR (1992) Preparation and decomposition of gold(I) hydrazido complexes: gold cluster formation. J Am Chem Soc 114 (4):1526-1527

88. Schulz-Dobrick M, Jansen M (2007) Characterization of Gold Clusters by Crystallization with Polyoxometalates: the Intercluster Compounds [Au9(dpph)4][Mo8O26], [Au9(dpph)4][PW12O40] and [Au11(PPh3)8Cl2]2[W6O19]. Z Anorg Allg Chem 633 (13-14):2326-2331

K. Konishi

89. Schulz-Dobrick M, Jansen M (2008) Intercluster compounds consisting of gold clusters and fullerides: [Au7(PPh3)7]C60 x THF and [Au8(PPh3)8](C60)2. Angew Chem Int Ed 47 (12):2256-2259

90. Gutrath B, Oppel I, Presly O, Beljakov I, Meded V, Wenzel W, Simon U (2013) [Au14(PPh3)8(NO3)4]: an example of a new class of Au(NO3)-ligated superatom complexes. Angew Chem Int Ed 52 (12):3529-3532

91. Kobayashi N, Kamei Y, Shichibu Y, Konishi K (2013) Protonation-Induced Chromism of Pyridylethynyl-Appended [core+exo]-Type Au Clusters. Resonance-Coupled Electronic Perturbation through pi-Conjugated Group. J Am Chem Soc 135 (43):16078–16081

92. Van der Linden JGM, Paulissen MLH, Schmitz JEJ (1983) Electrochemical reduction of the gold cluster Au9(PPh3)8

3+. Evidence for an ErErCr mechanism. Formation of the paramagnetic gold cluster Au9(PPh3)8

2+. J Am Chem Soc 105 (7):1903-1907 93. Smirnova ES, Echavarren AM (2013) A hexanuclear gold cluster supported by

three-center-two-electron bonds and aurophilic interactions. Angew Chem Int Ed 52 (34):9023-9026

94. Marsh RE (1984) Crystal structure of Au7(PPh3)7+: corrigendum. Inorg Chem 23 (22):3682-3682

95. Schulz-Dobrick M, Jansen M (2008) Synthesis and Characterization of Intercluster Compounds Consisting of Various Gold Clusters and Differently Charged Keggin Anions. Z Anorg Allg Chem 634 (15):2880-2884

96. Bhargava S, Kitadai K, Masashi T, Drumm DW, Russo SP, Yam VW, Lee TK, Wagler J, Mirzadeh N (2012) Synthesis and structures of cyclic gold complexes containing diphosphine ligands and luminescent properties of the high nuclearity species. Dalton Trans 41 (16):4789-4798

97. Pethe J, Strahle J (1999) Synthese und Kristallstruktur von [Au(AuNCO)(AuPPh3)8] Cl. Z Naturforsh B 54 (3):381-384

98. Pethe J, Maichle-Mössmer C, Strähle J (1998) Synthese und Struktur von K[Au(AuCl)(AuPPh3)8]](PF6)2. Z Anorg Allg Chem 624 (7):1207-1210

99. Nunokawa K, Onaka S, Yamaguchi T, Ito T, Watase S, Nakamoto M (2003) Synthesis and Characterization of the Au11 Cluster with Sterically Demanding Phosphine Ligands by Single Crystal X-ray Diffraction and XPS Spectroscopy. Bull Chem Soc Jpn 76 (8):1601-1602

100. Scherbaum F, Grohmann A, Huber B, Krüger C, Schmidbaur H (1988) “Aurophilicity” as a Consequence of Relativistic Effects: The Hexakis(triphenylphosphaneaurio)methane Dication[(Ph3PAu)6C]2 . Angew Chem Int Ed Engl 27 (11):1544-1546

101. Shichibu Y, Konishi K (2013) Electronic properties of [core+exo]-type gold clusters: factors affecting the unique optical transitions. Inorg Chem 52 (11):6570-6575

102. Jaw HRC, Mason WR (1991) Magnetic circular dichroism spectra for the octakis(triphenylphosphino)nonagold(3+) ion. Inorg Chem 30 (2):275-278

Documents

Phosphine-Coordinated Pure-Gold Clusters: Diverse ... · Phosphine-Coordinated Pure-Gold Clusters: Diverse Geometrical Structures and. . . Structurally defined molecular metal clusters